Ever since the United States became a world power, just over a hundred years ago, American foreign policy has been hammered out between two great forces, each deeply rooted in the American historical experience. One way of defining them is to think of them as the immigrant tradition, and the tradition of the frontiersman.

Most Americans are the descendants of men and women who, for whatever reason, consciously rejected a foreign country and chose to make a new life in the United States. Immigration peopled the United States with more or less resentful refugees from Britain, Europe and, later, from elsewhere. That massive element of the American experience has taught them to be wary of the outside world; to see it as treacherous, potentially slippery, cruel and morally suspect.

The opposing tradition is that of the frontier, which built into the American consciousness an expectation of steady expansion, manifest destiny, and at the same time a fear of the "other". It was, as Marxists used to say, not altogether an accident that it was precisely at the historical moment when the frontier closed that the United States began to embark on a career as world power; one that began with some fairly aggressive policies in the Caribbean and the annexation (more or less explicit) of Cuba, the Philippines, Hawaii, the Panama canal-zone and Haiti.

These two experiences, and the conscious and semi-conscious attitudes they have engendered, have set the template for American foreign policy. Of course, they have not been the only factors at work in shaping Washington’s responses. Other qualities have played their part at various times: generosity, a sense of national good fortune, religious impulses, inherited ethnic loyalties and antipathies, special interests, not to mention geopolitical theories and more or less rational fear.

But American action in the world has always reflected a primal ambivalence. On the one hand: a restless desire to acquire new territory, new markets, and new spheres of influence in which to preach the gospel of democracy. On the other: a reluctance to be soiled by the sordid connivings of a wicked world.

America the exceptional

There is, however, another factor that must be understood: American exceptionalism (although this belief is not universally held – the United States is after all a mature and tolerant society, almost uniquely able to criticise itself, and there are those who dissent even from exceptionalism).

Yet American exceptionalism is the key to the interaction between popular feeling and foreign policy. And it does not just mean that the United States has a higher average gross domestic product per capita, nor yet that it has the capacity to incinerate the northern hemisphere.

American exceptionalism maintains that the United States is morally superior to other societies; that it has a special, perhaps God-given destiny to bring to the world the benefits of peace and prosperity, capitalism and democracy, liberty and the pursuit of happiness.

For Americans, this idea has immense attraction and resonance. There are also many people around the world, though perhaps not as many as Americans assume, who prefer American society to the one in which they were born. “We have a fifth column in every country in the world”, a senior American politician once said to me. “It’s called the lower middle class.” Every civilised man, it used to be said, has two countries: France and his own. For better or for worse, that is now more generally true of the United States than of France.

For most non-Americans, however, the assumption of America’s moral superiority is not always so persuasive. After all, many other nations have thought they had a unique destiny, among them the British, French, Germans, Russians, Japanese, Chinese, and Spanish. This belief was sometimes comical, sometimes vicious. It has never done them any good in the long run. The further proposition - too easily arrived at by extension - that because American society is superior, so too individual Americans are morally superior. This is greeted with amusement by the confident, but with rage by the insecure.

Some part of the insanity that drives the suicide-bombers may come from this conflict: between envy of an imagined America, and resentment of the exceptionalist claim to superiority.

Securus judicat orbis terrarum, said Cardinal Newman: the world judges and is not judged. The United States does not permit itself that luxury (and it is one of the things I admire most about American civilisation).

There are Americans who believe it is their destiny to bring the real benefits of their civilisation to the world. There are others who suspect and resist that impulse.

Complex country, diverse world

Many Americans have a sense that it is the United States’s duty to save the world from the grossest forms of cruelty. In Kuwait, Bosnia, Kosovo, through the instrumentality of the so-called "CNN effect", this sense has also driven American administrations to attack dictatorial regimes in small countries.

But the United States is not a simple society. There is militarism, and there is also a real fear of militarism. There is patriotism (one can only admire the way American society - in spite of its divisions - has come together in the past two weeks), and also a latent contempt for the other. As a wise Englishman who lives in America said recently, those American flags are wonderful when they mean unity at home, frightening when seen from the outside.

One side of the American tradition says, “nobody defies us with impunity!” There is nothing new, nothing exceptional about that. The Latin motto of the Scots Guards is literally identical: it means, no one duffs me up and gets away with it. The difference is that the United States is now exceptionally able to punish its enemies. But there is another side to the American tradition: the knowledge that there are limits to the extent to which its power can be wisely and safely used.

The most dangerous consequence of American exceptionalism - and it lies at the heart of the concept of globalisation - is the belief that the whole of humanity is inevitably destined to adopt American culture and values. To fear this assumption, and to cherish diversity, implies no hostility to the best of that culture and those values. The British military historian Michael Howard expressed the point best: “The common western assumption that cultural diversity is a historical curiosity being rapidly eroded by the growth of a common, western-oriented, Anglophone world-culture shaping our basic values … is simply not true.”

If the tragedies in Manhattan and Washington succeed in challenging this assumption, it may be possible for American leaders to have a more realistic idea of what they can and cannot do.